Melt the butter over medium heat and add the scallions. Stir until scallions are wilted. Add the flour and whisk constantly for a minute or so. Slowly add the evaporated milk--about 1/4 of the can at a time--whisking to fully incorporate each addition.
Turn the heat to low. Add sherry, parsley, garlic powder, crab boil, cayenne pepper, and two pinches of salt and one pinch of pepper. Stir to combine, and continue to cook, stirring, until the sauce is thickened. This won't take more that a few minutes.
When the sauce is thick enough to coat a spoon, remove from the heat. Fold in the Swiss cheese and stir gently as it the cheese melts. Fold in the crab meat. Taste and correct seasoning.